Cells

• Prokaryotic cells


  Cells of bacteria and cyanobacteria (photosynthetic bacteria) that have no true nucleus but contain a single circular strand of DNA in the cytoplasm

  
• Eukaryotic cells


  Cells of animals, plants, fungi and protoctista that have several individual chromosomes found within a relatively large spherical nucleus bound by a nuclear membrane

  
• Kingdoms of living things


  • Animals
  • Plants
  • Fungi
  • Protoctista
  • Bacteria
  • Cyanobacteria
• Optical (light) microscopes


  • Combine two lenses (objective and eyepiece) to produce a much greater magnification
  • Can magnify up to x1400 - 1500
  • Limited by their resolving power/resolution
• Resolution


  The ability to distinguish between objects close together
• Magnification


  The degree to which something can be magnified or enlarged
• The wavelength of ordinary light is 2µm, which means that a light microscope can only distinguish between objects which are 2µm further apart from each other
• Advantages of light microscopes


  • Specimens can be living or dead
  • Specimens are not usually distorted
  • Specimen preparation is quick and simple
  • Natural colour is observed
  • Cheap and portable
• Electron microscopes


  • Use waves of electrons instead of light
  • Resolving power is much superior to light microscopes (1nm)
  • Magnification available is x500,000
  • Magnets are used to focus the beams of electrons
• Transmission electron microscopy


  Passes beams of electrons through a very thin object to see detail within a cell
• Scanning electron microscopy


  Bounces beams of electrons off the surface of an object to observe a 3D appearance
• Limitations of electron microscopy


  • Dehydration of the specimen causes distortion
  • Cannot examine living or wet specimens
  • Electrons cannot be directly viewed, must be projected onto a fluorescent screen or permanent record made on a photographic plate
  • From a thin specimen it is difficult to interpret cellular structures
  • No colour imaging
  • Pre-treatment may often result in artefacts / distorted structures
• 1m = 100cm = 1000mm = 1x10^6um = 1x10^9nm
• Plant cells


  • Protoplasts bordered by an extracellular cellulose cell wall

  Possess a vacuole and chloroplasts

  Lack centrioles
• Animal cells


  • Lack chloroplasts

  Possess centrioles
• Cell surface membrane


  Also known as the plasma membrane

  Serves as a boundary between the cell and its environment

  Consists of phospholipid and protein molecules arranged in a fluid-mosaic structure

  
• Structure of cell surface membrane


  • Bilayer of phospholipids with polar phosphate ends outermost and non-polar hydrocarbon chains innermost

  May contain glycoproteins/glycolipids

  Some proteins are peripheral (extrinsic) while some are integral (intrinsic) and span the membrane

  Proteins have hydrophobic regions in contact with lipid layer and hydrophilic regions facing out

  Carbohydrate is found on the outer face only

  Cholesterol may be present to act as a temperature buffer

  
• Functions of cell surface membrane


  Lipids give the membrane fluidity

  Allows membranes to fuse during exocytosis and endocytosis

  Prevents passage of water soluble substances while allowing passage of fat soluble molecules

  Some membrane proteins act as carriers moving molecules actively across the membrane against a concentration gradient, using ATP

  
• Facilitated diffusion


  The passive movement of molecules down a concentration gradient through special carrier proteins in the cell membrane
• Glycocalyx


  The extending of the outer phospholipid layer of the membrane, containing polysaccharides bound to membrane proteins (glycoproteins) or phospholipids (glycolipids)

  
• Mitochondria


  • Sausage-shaped organelles found in all cells

  Involved in the production of ATP by aerobic respiration

  Possess an outer and inner membrane, with the inner membrane folded into cristae to increase surface area

  Matrix contains enzymes, lipids, traces of DNA and small ribosomes

  
• Ribosomes


  Non-membranous organelles found in the cytoplasm

  Around 20nm in diameter in eukaryotic cells (80s type) and slightly smaller in prokaryotic cells (70s type)

  May occur freely, in groups (polyribosomes), or associated with the endoplasmic reticulum

  Made up of rRNA and protein, consist of one large and one small subunit

  Move along mRNA to facilitate protein synthesis

  
• Endoplasmic reticulum (ER)


  Formed by the outward folding of the outer membrane of the nuclear envelope

  Rough ER (RER) is associated with ribosomes and is the site of protein synthesis

  Smooth ER (SER) is concerned with lipid synthesis

  Overall role is to collect and transport synthesised products within the cell

  
• Nucleus


  Contains DNA, which is condensed into chromosomes during cell division and spread throughout as chromatin (DNA bound to histones) at other times

  Contains one or more nucleoli, which are not distinct organelles but contain DNA involved in ribosome synthesis

  
• Endoplasmic reticulum (ER)


  Rough ER (RER) - involved in protein synthesis, Smooth ER (SER) - involved in lipid synthesis
• Endoplasmic reticulum (ER)


  • Collects and transports synthesised products within the cell
  • Small vesicles bud off the ER to form the forming face of the Golgi apparatus

  
• Nucleus


  • Contains DNA
  • Bordered by a double membrane/envelope with pores
  • Pore to allow the movement of large molecules between nucleoplasm and cytoplasm
  • The outer membrane of the envelope is evaginated (outfolded) to form the ER
  • Stores the cell's genetic matter/chromatin
  • Contains the nucleolus which is the site of the synthesis of ribosomal RNA
  • Euchromatin (light) is the site of transcription of mRNA/rRNA
  • Site of DNA replication
  • Euchromatin (light) is active (since it is unwound) while heterochromatin (dark) is inactive (and thus more dense)

  
• Golgi apparatus


  • Composed of stacks of flattened membrane bound sacs known as cisternae
  • At the forming face, new cisternae are constantly being formed by the fusion of vesicles (probably derived from the RER)
  • At the maturing face, the cisternae break into vesicles once more
  • Proteins are processed and packaged as they travel throughout the cisternae
  • Proteins can be joined together to produce quaternary proteins, fold into secondary and tertiary structures, and have carbohydrates added to form glycoproteins
  • These are then packaged into vesicles for secretion from the cell

  
• Lysosomes


  • Produced by the Golgi bodies
  • Contain powerful hydrolytic enzymes known as lysozymes
  • Have thickened membranes to prevent accidental leakage
  • Digest degenerate organelles
  • May release their enzymes, thereby digesting the whole cell (autolysis)
  • May fuse with phagosomes containing engulfed/phagocytosed material

  
• Vesicles


  • Membrane bound sacs responsible for endocytosis and exocytosis
  • Formed by budding of Golgi and ER and invagination of the cell surface membrane
  • Can contain modified protein for transport to plasma membrane/export out of the cell (exocytosis)

  
• Endocytosis


  • Transport of materials into the cytoplasm which are too big to be transported by membrane carrier molecules
  • Two types: phagocytosis (cell eating) and pinocytosis (cell drinking)

  
• Exocytosis


  Secretion of substances from the cell, the reverse of endocytosis
• Chloroplasts


  • Bounded by a double membrane known as the chloroplast envelope
  • The outer membrane is similar in structure to the plasma membrane while the inner membrane is folded into a series of structures known as lamellae
  • The stroma is a colourless matrix in which are embedded structures like stacks of coins called grana
  • Each granum is made up of 100-200 closed flattened sacs called thylakoids
  • The photosynthetic pigments such as chlorophyll are located within the thylakoids
  • Also present are starch grains (large) which act as a temporary store for the products of photosynthesis, and a small amount of DNA

  
• Microtubules


  • Hollow cylinders formed from the protein tubulin
  • Form the cytoskeleton of the cell which maintains its shape and keeps the organelles in place
  • The spindle fibres that appear during nuclear division are microtubules that grow from the centrioles and are responsible for the movement of chromosomes
  • Also found in cilia and flagella

  
• Cell wall


  • Entirely outside the cell surrounding the cell membrane
  • Plant cell walls are primarily constructed of cellulose
  • Cellulose is laid down as microfibrils, each consisting of many cellulose molecules cross linked to each other
  • The primary cell wall is made up of many microfibrils orientated in different random directions, allowing the cell to expand as it grows
  • The secondary cell wall forms when the cell reaches full size, with additional layers of cellulose deposited in the same direction in each layer but different between layers, creating lattice type strength
  • Middle lamellae links adjacent cells and is largely made of polysaccharides called pectin
  • Tiny strands of cytoplasm called plasmodesmata pass through holes in the walls and connect each plant cell to its neighbour both physically and metabolically
• Vacuole


  • Contains ions and water and plays an important role in turgor support
  • Surrounded by a single membrane called a tonoplast

  
• Fungal cells


  • Made from eukaryotic cells
  • Have cell walls made from chitin, not cellulose
  • Possess a vacuole like plant cells but cannot photosynthesize, so lack chloroplasts
  • Similarities to animal cells include glycogen store and lysosomes
  • Possess a nucleus or multiple nuclei
  • Form long strand-like structures called hyphae which spread through the substance the fungi grow on

  
• Homogenisation


  Tissue is blitzed or ground to produce components of a similar size and release the contents of the cell
• Centrifugation


  1. Used to separate organelles of a cell based on their mass after the cells have been homogenised

  2. Involves spinning a sample at very high speeds in a liquid medium which allows components of different mass to move to the top or bottom of the medium

  3. Heavy components settle to the bottom forming a pellet, lighter organelles float in the liquid known as the supernatant

  4. By repeatedly removing the pellet and spinning the supernatant for longer, smaller and smaller organelles can be selected for

  
• Ribosomes synthesize proteins using instructions from DNA.